1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly, to a semiconductor device, such as a MOSFET, which has a gate insulating film exhibiting a specific dielectric constant distribution, and a method of manufacturing the same.
2. Description of the Related Art
MOSFETs formed on a semiconductor substrate are getting smaller. The gate insulating film of a MOSFET has become proportionally thinner. The roadmap of ITRS (International Technology for Roadmap Semiconductors), U.S.A., predicts that the oxide films for gate insulating films will have a thickness of 1.2 nm or less in the middle of 2002.
A gate leakage current, which results from a direct tunneling current, flows in such thin oxide films in great quantities. This leads to an increase in power consumption in MOSFETs having such an oxide film. In some cases, this may decreases the operating efficiency of MOSFETs. The direct tunneling current depends on the thickness of the gate insulating film. The thickness of the gate insulating film can be decreased, but to a limited value.
To solve the problems that small MOSFETs have due to the direct tunneling current, it is proposed that the gate insulating film of each small MOSFET should have a higher dielectric constant than that of silicon oxide film, i.e., the conventional gate insulating film. More specifically, it is proposed that the gate insulating film be made of, for example, SixNy, TaxOy, TixOy, HfxOy, ZrxOy or the like. The gate insulating film can then have a sufficient effective thickness, i.e., physical thickness, without having its electrically equivalent thickness to SiO2 increased. If a gate insulating film having so high dielectric constant is used, however, the thermal stability and resistance to oxidation of the film must be taken into consideration in the process of manufacturing the small MOSFET.
Generally, films having high dielectric constant are inferior to the conventional silicon oxide film, in terms of thermal stability and resistance to oxidation. Therefore, it is well anticipated that if such a film having a high dielectric constant is used as a gate insulating film, the edge of the gate electrode (made of, for example, polysilicon) cannot be easily rounded by post oxidation in order to suppress the electric field concentration at the gate edge of the MOSFET.
In the future the gate electrode of the small MOSFET will probably be made of metal, not polysilicon. The edge of a gate electrode made of metal will hardly be rounded by means of post oxidation.
In the conventional semiconductor device, the gate edge cannot be rounded due to the limited thermal stability and resistance to oxidation of the gate insulating film if the gate insulating film of the small MOSFET is a film having a high dielectric constant as described above. The gate insulating film may have but low reliability due to, for example, a small breakdown voltage, which results from the electric field concentration at the gate edge.